1. Field of the Invention
The present invention relates to an upstream signal control apparatus for stabilizing an upstream signal transmitted from a terminal in a cable television system (to be referred to as a CATV system hereinafter) having a two-way communication function.
2. Description of the Related Art
In addition to a broadcasting function based on television signals, a CATV system has a two-way communication function which allows communication between a broadcasting center and terminals. A CATV system having a two-way communication function is called a two-way addressable CATV system. The two-way communication function enables communication of data between a broadcasting center and terminals. For example, data stored in a terminal can be erased, updated, or read by transmitting a downstream signal including command data from the broadcasting center. In addition, questionnaire data, public opinion survey data, or the like can be transmitted as an upstream signal from a terminal to the broadcasting center in response to a command therefrom. When an upstream signal is to be transmitted from a terminal to a broadcasting center by using such a two-way communication function, the upstream signal preferably has a sufficient level when it reaches the broadcasting center.
In a two-way addressable CATV system, therefore, when a new terminal is installed, the transmission level of an upstream signal to be transmitted from the terminal to the broadcasting center is adjusted. The transmission level of an upstream signal from a terminal is adjusted by the following two methods.
In the first method, when a terminal is installed, the degree of attenuation of the level of a downstream signal transmitted from the broadcasting center to the terminal is measured. The degree of attenuation of an upstream signal to be transmitted from the terminal to the broadcasting center is estimated from the degree of attenuation of the downstream signal. When the degree of attenuation of the upstream signal is estimated, the transmission level of the upstream signal is adjusted to fall within a predetermined range upon arrival of the upstream signal at the broadcasting center.
In the second method, an upstream signal detector is arranged in the broadcasting center. The upstream signal detector detects the level of an upstream signal from a terminal. When the level of the upstream signal is low, the broadcasting center transmits a transmission level control signal to the corresponding terminal so as to control its transmission level. With this operation, the transmission level of the upstream signal from the terminal is automatically controlled.
Both the methods, however, have the following drawbacks.
According to the first method, since the transmission level of a terminal is fixed once it is set, if, for example, there are drifts due to changes in temperature and fluctuations in signal level of trunk line systems due to changes in quality over time, variations in level of upstream signals from the respective terminals are increased at the broadcasting center. In addition, if the level of an upstream signal from a certain terminal falls outside an allowable range, an operator in the broadcasting center must go to a corresponding subscriber's house to adjust the transmission level again.
According to the second method, the transmission level of an upstream signal from each terminal is automatically controlled, and hence it is convenient. However, as the number of subscribers is increased, the communication time required to adjust the transmission levels of the terminals is prolonged. This means that a ratio of the communication time for adjustment to the total data communication time of the system is increased, and that the data communication speed is relatively decreased. If an adjustment frequency of the transmission levels of upstream signals is decreased in order to prevent such inconvenience, variations in transmission levels of terminals are increased.